Modulated reflectance measurement system using UV probe

ABSTRACT

A modulated reflectance measurement system includes lasers for generating an intensity modulated pump beam and a UV probe beam. The pump and probe beams are focused on a measurement site within a sample. The pump beam periodically excites the measurement site and the modulation is imparted to the probe beam. For one embodiment, the wavelength of the probe beam is selected to correspond to a local maxima of the temperature reflectance coefficient of the sample. For a second embodiment, the probe laser is tuned to either minimize the thermal wave contribution to the probe beam modulation or to equalize the thermal and plasma wave contributions to the probe beam modulation.

PRIORITY CLAIM

The present application claims priority to U.S. Provisional PatentApplication Ser. No. 60/413,229, filed Sep. 23, 2002 and U.S.Provisional Patent Application Ser. No. 60/413,094, filed Sep. 24, 2002,both of which are incorporated in this document by reference.

TECHNICAL FIELD

The subject invention relates generally to optical methods forinspecting and analyzing semiconductor wafers and other samples. Inparticular, the subject invention relates to methods for increasing theaccuracy and flexibility of systems that use photo modulatedreflectivity to analyze semiconductor wafers.

BACKGROUND OF THE INVENTION

There is a great need in the semiconductor industry for metrologyequipment that can provide high resolution, nondestructive evaluation ofproduct wafers as they pass through various fabrication stages. Inrecent years, a number of products have been developed for thenondestructive evaluation of semiconductor samples. One such product hasbeen successfully marketed by the assignee herein under the trademarkTherma-Probe. This device incorporates technology described in thefollowing U.S. Pat. Nos. 4,634,290; 4,646,088; 5,854,710; 5,074,669 and5,978,074. Each of these patents is incorporated herein by reference.

In the basic device described in the patents, an intensity modulatedpump laser beam is focused on the sample surface for periodicallyexciting the sample. In the case of a semiconductor, thermal and plasmawaves are generated in the sample that spread out from the pump beamspot. These waves reflect and scatter off various features and interactwith various regions within the sample in a way that alters the flow ofheat and/or plasma from the pump beam spot.

The presence of the thermal and plasma waves has a direct effect on thereflectivity at the surface of the sample. As a result, subsurfacefeatures that alter the passage of the thermal and plasma waves have adirect effect on the optical reflective patterns at the surface of thesample. By monitoring the changes in reflectivity of the sample at thesurface, information about characteristics below the surface can beinvestigated.

In the basic device, a second laser is provided for generating a probebeam of radiation. This probe beam is focused collinearly with the pumpbeam and reflects off the sample. A photodetector is provided formonitoring the power of reflected probe beam. The photodetectorgenerates an output signal that is proportional to the reflected powerof the probe beam and is therefore indicative of the varying opticalreflectivity of the sample surface. The output signal from thephotodetector is filtered to isolate the changes that are synchronouswith the pump beam modulation frequency. A lock-in detector is typicallyused to measure both the in-phase (I) and quadrature (Q) components ofthe detector output. The two channels of the output signal, namely theamplitude A²=I²+Q² and phase Θ=arctan (I/Q) are conventionally referredto as the photomodulated reflectivity (PMR) or Thermal Wave (TW) signalamplitude and phase, respectively.

Dynamics of the thermal- and carrier plasma-related components of thetotal PMR signal in a semiconductor is given by the following generalequation:

$\frac{\Delta\; R}{R} = {\frac{1}{R}\left( {{\frac{\partial R}{\partial T}\Delta\; T_{0}} + {\frac{\partial R}{\partial N}\Delta\; N_{0}}} \right)}$

where ΔT₀ and ΔN₀ are the temperature and the carrier plasma density atthe surface of a semiconductor, R is the reflectance, dR/dT is thetemperature reflectance coefficient and dR/dN is the carrier reflectancecoefficient. For silicon, dR/dT is positive in the visible and near-UVpart of the spectrum while dR/dN remains negative throughout the entirespectrum region of interest. The difference in sign results indestructive interference between the thermal and plasma waves anddecreases the total PMR signal. The magnitude of this effect depends onthe nature of a semiconductor sample and on the parameters of thephotothermal system, especially on the pump and probe beam wavelengths.

In the early commercial embodiments of the TP device, both the pump andprobe laser beams were generated by gas discharge lasers. Specifically,an argon-ion laser emitting a wavelength of 488 nm was used as a pumpsource. A helium-neon laser operating at 633 nm was used as a source ofthe probe beam. Lore recently, solid state laser diodes have been usedand are generally more reliable and have a longer lifetime than gasdischarge lasers. In the current commercial embodiment, the pump laseroperates at 780 nm while the probe laser operates at 670 nm.

In practice, the use of the visible spectrum for both pump and probebeams has proven to be effective for a broad range of practicalapplications. Alternatively, U.S. Pat. Ser. No. 5,034,611 discloses aPMR system having a 488 nm pump beam and a beam probe in the UV range of200 through 345 nm. That particular combination is believed to be aneffective tool for measuring implantation doses above 10¹⁵ cm⁻² atrelatively shallow depth (i.e., approximately 10 nm).

As may be appreciated, it is entirely possible to construct PMR systemsthat operate at probe and pump wavelengths that differ from the systemsdescribed above. As will be described below, there are applications thatbenefit from these alternate configurations. This is particularly truefor applications that involve relatively high temperature reflectancecoefficients.

SUMMARY

The present invention provides a modulated reflectance measurementsystem with the capability to make measurements using a probe beam inthe near-UV and UV parts of the spectrum. The measurement systemincludes a probe laser and a pump laser, each producing monochromaticlight at a different spectrum. A modulator causes the pump laser to havean intensity modulated output, referred to as the pump beam. The probelaser produces an output that is typically non-modulated (i.e., constantintensity). This output is referred to as the probe beam.

The output of the probe laser and the output of the pump laser arejoined into a collinear beam using a laser diode power combiner. Anoptical fiber transports the now collinear probe and pump beams from thelaser diode power combiner to a lens or other optical device forcollimation. Once collimated, the collinear beam is focused on a sampleby an objective lens.

A reflected portion of the collinear probe and pump beams is redirectedby a beam splitter towards a detector. The detector measures a portionof the probe beam which is reflected by the sample and forwards acorresponding signal to a filter. The filter typically includes alock-in amplifier that uses the output of the detector, along with theoutput of the modulator to produce quadrature (Q) and in-phase (I)signals for analysis. A processor typically converts the Q and I signalsto amplitude and/or phase values to analyze the sample. In other cases,the Q and I signals are used directly.

For one implementation, the probe laser is configured to operate in the400–405 nm wavelength range. At that spectral range, the differencebetween the temperature reflectance coefficient and the carrierreflectance coefficient is maximized. This increases the signal measuredby the detector when measuring samples with thermally-dominatedmodulated reflectance signals. For a second implementation, the probelaser operates in the 360 nm wavelength range. At that spectral range,both the temperature and carrier plasma reflectance coefficients havethe same (negative) sign leading to a constructive interference betweenthe thermal and carrier plasma wave contributions to the total PMRsignal. Once again, this increases the signal measured by the detectorwhen measuring samples with thermally-dominated modulated reflectancesignals.

A third implementation of the modulated reflectance measurement systemuses a wavelength tunable probe laser. The probe laser operates at anominal or central wavelength in the UV range and is tunable to operateat shorter or longer wavelengths. To analyze a sample, an initial siteis measured within the sample. At the initial site, the probe laser istuned to minimize the thermal wave contribution to the total PMR signal.At subsequent sites, the probe laser wavelength is held constant and thePMR signal re-measured. Differences in the measured signal indicatedifferences in sample characteristics. This technique is effective forsamples that exhibit a thermally-dominated PMR signal.

For samples where the PMR signal is not thermally-dominated, the probelaser is tuned to equalize the contributions from the thermal wave andcarrier plasma to the total PMR signal. This allows the modulatedreflectance measurement system to operate in the plasma-to-thermaltransition region which maximizes the sensitivity of PMR phasemeasurements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a modulated reflectance measurement systemas provided by an embodiment of the present invention.

FIG. 2 is a graph showing the temperature reflectance coefficient of anidealized sample plotted as a function of measurement wavelength.

FIG. 3 is a graph showing the temperature reflectance coefficients ofamorphous silicon and crystalline silicon plotted as a function ofmeasurement wavelength.

FIG. 4 is a graph plotting thermal wave amplitude and thermal wave phaseas a function of implantation dose.

FIG. 5 is a detailed view of the plasma-to-thermal transition regionoriginally shown in FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides a modulated reflectance measurementsystem with the capability to make measurements using a probe beam inthe near-UV and UV spectrum. The modulated reflectance measurementsystem is adaptable for use with fixed or tunable wavelength probebeams. One possible implementation for the modulated reflectancemeasurement system is shown in FIG. 1 and designated 1001 As shown,modulated reflectance measurement system 100 includes a probe laser 102and a pump laser 104. Each laser 102, 104 is typically monochromatic andeach laser 102, 104 typically operates at a different spectrum. Lasers102, 104 are generally diode-based or diode-pumped semiconductor lasers.Solid state laser diodes are available that have outputs throughout theentire visible spectrum as well as in the infrared and near UV. Lasers102, 104 are controlled by a processor 106 and a modulator 108.Modulator 108 causes pump laser 104 to have an intensity modulatedoutput, referred to as the pump beam. Probe laser 102 produces an outputthat is typically non-modulated (i.e., constant intensity). This outputis referred to as the probe beam.

As the probe beam leaves probe laser 102, it is preferably collected byan optical fiber 110. Optical fiber 110 is typically single mode anddirects the probe beam through a dichroic mirror 112 towards a sample114. Sample 114 is positioned on an X-Y stage 116 allowing sample to bemoved in translation relative to the probe beam. As the pump beam leavespump laser 104, it is preferably collected by a second optical fiber118. Optical fiber 118 is typically single mode and directs the pumpbeam to a tracking mechanism 1201 After leaving tracking mechanism 120,the pump beam is redirected by dichroic mirror 112. The redirectionaligns the pump beam to be collinear with the probe beam as the probebeam travels towards sample 114.

After striking sample 114, the reflected pump and probe beam areredirected by a beam splitter 122 through a filter 124 and towards adetector 126. Filter 124 removes the pump beam component of thereflected beams, while allowing the probe beam component to passuninhibited. Detector 126 provides an output signal that is proportionalto the power of the reflected probe beam. Detector 126 is arranged to beunderfilled so that its output can be insensitive to any changes in beamdiameter or position. In the preferred embodiment, detector 126 is aquad cell generating four separate outputs. When used to measurereflected beam power, the output of all four quadrants are summed. Asdescribed in U.S. Pat. No. 5,978,074, the apparatus can also be operatedto measure beam deflection. In the latter case, the output of oneadjacent pair of quadrants is summed and subtracted from the sum of theremaining pair of quadrants.

The output of detector 126 is passed to a filter 128. Filter 128typically includes a lock-in amplifier that uses the output of detector126, along with the output of modulator 108 to produce quadrature (Q)and in-phase (I) signals for analysis. Processor 106 typically convertsthe Q and I signals to amplitude and/or phase values to analyze thesample. In other cases, the Q and I signals are used directly.

As an alternative to using an electronic heterodyne down-mixing system,it is also possible to reduce the frequency of detection using anoptical heterodyne approach. Such an optical approach is disclosed inU.S. Pat. No. 5,408,327, incorporated herein by reference. In the lattersystem, both of the laser beams are modulated but at slightly differentfrequencies. Both beams generate thermal and plasma waves at theirrespective modulation frequencies. The beam from one laser picks up anintensity modulation upon reflection due to the modulated opticalreflectivity induced in the sample by the other beam. The PMR signalpicked up upon reflection “mixes” with the inherent modulation of thebeam, creating additional modulations in the beam at both the sum anddifference frequency. This process is analogous to electricalheterodyning. The difference or “beat” frequency is much lower thaneither of the initial beam modulation frequencies and can therefore bedetected by a low frequency lock-in amplifier.

To insure proper repeatability of the measurements, the signals must benormalized in processor 1061 Accordingly, and as discussed in the aboveidentified patents, in the preferred embodiment, a variety of referencedetectors would be provided, the outputs of which are used to normalizethe output of detector 126. Other optical elements, such as filters,collimators, shutters and steering optics would be included, all ofwhich are all well known to those skilled in the art.

In some measurements, the two beams will be positioned so that the spotswill overlap on the sample surface. In addition, measurements can betaken at various spacings between the pump and probe beam spots.Measurements at different spatial separations are discussed in greaterdetail in U.S. Pat. No. 5,978,074.

As noted above, there are many different thermal/plasma wave measurementtechniques besides the measurement of modulated optical reflectivity.These devices are described in the above-cited patents and includemeasurement of the angular deviations of the probe beam as well asinterferometric techniques.

Information about such systems can be found in U.S. Pat. Nos. 4,521,118;5,522,510; 5,298,970; 6,411,390 and 6,268,916 all of which areincorporated herein by reference. Such systems for monitoring thevariations of a probe beam are within the scope of the subjectinvention.

The pump beam generates both carrier plasma and thermal waves in asample 114. Detector 126 measures the modulated reflectance, which isproportional to the nonradiative energy, produced by both the plasma andthermal waves. The modulated reflectance is described by the followingequation:

$\frac{\Delta\; R}{R} = {\frac{1}{R}\left( {{\frac{\partial R}{\partial T}\Delta\; T_{0}} + {\frac{\partial R}{\partial N}\Delta\; N_{0}}} \right)}$

-   -   where ΔT₀ and ΔN₀ are the temperature and the carrier plasma        density at the surface of a semiconductor, R is the reflectance,        dR/dT is the temperature reflectance coefficient and dR/dN is        the carrier reflectance coefficient. For silicon, dR/dT is        positive in the visible and near-UV part of the spectrum while        dR/dN remains negative throughout the entire spectrum region of        interest. This difference in signs decreases the total PMR        signal due to partial compensation of changes in reflectance        caused by the thermal and carrier plasma waves.        Fixed Wavelength Operation

As shown in FIG. 2, dR/dT increases dramatically in the near-UVwavelength range 390–410 nm and decreases dramatically in the UVwavelength range around 360 nm. Although not shown, dR/dN remainspractically unchanged over this range of wavelengths. Forimplementations where probe laser 104 operates at a fixed wavelength,the relationship between the dR/dT and dR/dN waveforms results in twodesirable operating modes for modulated reflectance measurement system100. For the first, probe laser 102 operates in the 400–405 nmwavelength range. At that spectral range, the difference between dR/dTand dR/dN increases the signal measured by detector 126 by as much as afactor of ten when compared to conventional PMR systems when measuringsamples with thermally-dominated modulated reflectance signals.

For the second operating mode of modulated reflectance measurementsystem 100, probe laser 102 operates in the 360 nm wavelength range. Atthat spectral range, both the temperature and carrier plasma reflectancecoefficients would be of the same (negative) sign leading to aconstructive interference between the thermal and carrier plasma wavecontributions to the total PMR signal. As a result, the overall PMRsignal is expected to increase even more than one would anticipate fromthe increased thermal component (higher absolute value of dR/dT). ThisUV spectral region could be beneficial for almost all conventional TPapplications including those where the signal is dominated by thecarrier plasma effects, such as junction depth measurements in implantedand annealed semiconductors.

Tunable Wavelength Operation

With appropriate modifications, it is possible to configure modulatedreflectance measurement system 100 to be wavelength tunable. For thistype of configuration, probe laser 104 produces a probe beam at anominal or central wavelength λ₀ within the UV spectrum. Probe laser 104is tunable around λ₀ in order to be able to turn the thermal wavecomponent of the PMR signal on and off and/or to maximize its amplitude.

As shown in FIG. 3, wavelength dependencies for the thermal reflectioncoefficients for the crystalline (c-Si) and amorphous (α-Si) siliconexhibit a nonmonotonic behavior in the near-UV and UV parts of thespectrum. In the spectral range between ˜370 nm (negative maxima ofdR/dT) and ˜405 nm (positive maxima of dR/dT) both dependencies cross azero line. At these inflection points the amplitude of the thermal wavecomponent is equal to zero, i.e., is fully compensated. The insert inFIG. 3 shows schematically an enlarged view of a family of dR/dT curvescorresponding to Si samples implanted with different doses (D₁ and D₂)near the zero line. With curves for α-Si (completely amorphous) and c-Si(non-implanted) being the two extreme cases, all measurement pointscorresponding to different samples with different implantation doses (orenergies) will fall in between these two extremes.

In practical measurements, the probe beam wavelength is first tuned tocompensate for the thermal wave component for the sample implanted withthe first dose D₁. Then the second measurement of the sample with doseD₂ is taken at the same wavelength. If D₁ and D₂ are different, thethermal wave component will appear indicating differences in samplecharacteristics. The sensitivity to implantation dose in suchcompensation measurements can be significantly higher than that for theconventional TP methodology at any other probe beam wavelengths.

This compensation technique will be effective only for the samplesexhibiting a thermally-dominated behavior of the PMR signal.Unfortunately, ion-implanted semiconductors do not always exhibit thistype of behavior. FIG. 4 illustrates schematically a typical PMR signalamplitude (left scale) and phase (right scale) dose behavior. As can beappreciated, the PMR amplitude and phase dose dependencies exhibitdifferent behavior in different parts of the dose spectrum depending onthe physical mechanism dominating the PMR signal—thermal wave or carrierplasma.

At low implantation doses (the region marked “I” in FIG. 4), the signalis plasma-dominated while at intermediate and high implantation doses(the regions “III” and “VI”) thermal wave component dominates thesignal. In the region marked “II”, the amplitude of the carrier plasmawave component is commensurate with that of the thermal one and, beingof the opposite sign for the set of the pump/probe beam wavelengths fromthe visible part of the spectrum, results in a sharp negative peak in aplasma-to-thermal transition region. In region II, the PMR phaseexperiences a sharp (near 180°) drop indicating a change in the dominantphysical mechanism behind the PMR signal from plasma to thermal.Dynamics of the carrier plasma wave and thermal components of the TWsignal in this region is shown in more details in FIG. 5.

As shown in FIG. 4, PMR phase is a sensitive parameter only in theplasma-to-thermal transition region (i.e., when the thermal wave andplasma component amplitudes are comparable) and carries almost noinformation outside this region. To take advantage of PMR phasesensitivity in the plasma-to-thermal transition region, modulatedreflectance measurement system 100 is configured so that probe laser 104is tunable in the UV to near-UV spectral range (370–410 nm). Duringmeasurement, this is used to equalize the contributions from the thermalwave and carrier plasma to the total PMR signal. This effectively shiftsthe dose position of this transition region by changing relativeamplitudes of the carrier plasma and thermal wave components. As aresult, it becomes possible to measure and analyze PMR phase informationfor a much broader range of dosages. In a photothermal system havingmore than two lasers it would then be possible to shift a very sensitiveTW phase drop back and forth on implantation dose scale depending on thesample under investigation.

In general, it should be appreciated that the implementation of FIG. 1is intended to be representative in nature. The use of UV probe lasers,both fixed and tunable is possible with a range of measurement systemsand is not limited to the specific combination of components shown inFIG. 1.

1. A method for evaluating a sample, the method comprising: generatingan intensity modulated pump beam; generating a probe beam at awavelength within the UV range that corresponds to a local maxima of thetemperature reflectance coefficient of the sample; focusing the pumpbeam on the sample to periodically excite a region of the samplesurface; focusing the probe beam within the periodically excited region;monitoring a portion of the probe beam that is reflected by theperiodically excited region and generating corresponding output signals;and evaluating the sample by analyzing the detector output signals.
 2. Amethod for evaluating a sample, the method comprising: focusing anintensity modulated pump beam on the sample surface to periodicallyexcite a first measurement site; focusing an ultra-violet probe beamwithin the first measurement site; measuring the modulation imparted tothe probe beam by the first measurement site; and tuning the wavelengthof the probe beam to minimize the thermal wave contribution to the probebeam modulation.
 3. A method as recited in claim 2 that furthercomprises the steps of: focusing the pump beam on the sample surface toperiodically excite a second measurement site; focusing the ultra-violetprobe beam within the second measurement site; measuring the modulationimparted to the probe beam by the second measurement site; and comparingthe modulations imparted by the first and second measurement sites.
 4. Amethod for evaluating a sample, the method comprising: focusing anintensity modulated pump beam on the sample surface to periodicallyexcite a first measurement site; focusing an ultra-violet probe beamwithin the first measurement site; measuring the modulation imparted tothe probe beam by the first measurement site; and tuning the wavelengthof the probe beam to vary the thermal and plasma wave contributions tothe probe beam modulation.
 5. A method as recited in claim 4, whereinthe wavelength of the probe beam is varied to equalize the thermal andplasma wave contributions to the probe beam modulation.
 6. A device forevaluating a sample, the device comprising: a first illumination sourceproducing an intensity modulated pump beam for periodically exciting aregion on the sample; a second illumination source producing a singleprobe beam to reflect off the region on the sample surface that has beenperiodically excited where the probe beam has a spectral range selectedfrom a group that includes: 395 to 410 nm and 355 to 365 nm; optics forcombining the pump and probe beams so that the pump and probe beamspropagate collinearly to the sample; a detector for monitoring themodulated changes in the power of the single reflected probe beam andgenerating output signals in response thereto, said output signalscorresponding to the modulated optical reflectivity of the sample; and aprocessor for evaluating the sample using the detector output signals.7. A device as recited in claim 6, wherein the spectral range of theprobe beam is 400 to 405 nm.
 8. A device for evaluating a sample, thedevice comprising: a first illumination source producing an intensitymodulated beam for periodically exciting a region on the sample; asecond illumination source producing a probe beam to be reflected by theperiodically excited region, where the probe beam has a wavelengthwithin the UV range that corresponds to a local maxima of thetemperature reflectance coefficient of the sample; a detector formonitoring the modulated changes in the reflected probe beam andgenerating output signals in response thereto; and a processor forevaluating the sample by analyzing the detector output signals.
 9. Adevice for evaluating a sample, the device comprising: a firstillumination source producing an intensity modulated beam forperiodically exciting a region on the sample; a second illuminationsource producing a probe beam to be reflected by the periodicallyexcited region, where the wavelength of the probe beam is tunable tominimize the thermal wave contribution to the probe beam modulation; adetector for monitoring the modulated changes in the reflected probebeam and generating output signals in response thereto; and a processorfor evaluating the sample by analyzing the detector output signals. 10.A device for evaluating a sample, the device comprising: a firstillumination source producing an intensity modulated beam forperiodically exciting a region on the sample; a second illuminationsource producing a probe beam to be reflected by the periodicallyexcited region, where the wavelength of the probe beam is tunable toequalize the thermal and plasma wave contributions to the probe beammodulation; a detector for monitoring the modulated changes in thereflected probe beam and generating output signals in response thereto;and a processor for evaluating the sample by analyzing the detectoroutput signals.
 11. A device for evaluating a sample, the devicecomprising: a first illumination source producing an intensity modulatedbeam for periodically exciting a region on the sample; a secondillumination source producing a probe beam to be reflected by theperiodically excited region, where the wavelength of the probe beam istunable to vary the thermal and plasma wave contributions to the probebeam modulation; a detector for monitoring the modulated changes in thereflected probe beam and generating output signals in response thereto;and a processor for evaluating the sample by analyzing the detectoroutput signals.